A method of changing characteristics of a motor by changing formation of windings of the motor is conventionally known. FIGS. 6a and 6b show a method of changing characteristics of a motor by connecting windings of the motor as a Y-connection or a .DELTA.-connection. The larger number of turns the winding for each phase has, the larger torque can be produced with low speed rotation. Therefore, when large torque is needed with low speed rotation, the Y-connection (in which terminals X, Y, Z are connected together while power lines of three phases are connected to terminals U, V, W, respectively) as shown in FIG. 6a is selected. (This formation will be referred to as "low speed winding formation".) When high speed rotation is needed, the .DELTA.-connection (in which terminals U and Z, V and X, W and Y are connected respectively and power lines are connected to the respective connecting points) as shown in FIG. 6b is selected. (This formation will be referred to as "high speed winding formation".)
According to another known method, as shown in FIGS. 7a and 7b, a Y-connection of three windings for three phases of a motor is formed with terminals U2, V2, W2 provided at intermediate portions of the three windings. For low speed rotation, as shown in FIG. 7a, power lines of three phases are connected to terminals U1, V1, W1 to thereby use the three entire windings for three phases. (This formation will be referred to as "low speed winding formation".) (It is to be noted that used parts of windings are depicted with bold lines.) For high speed rotation, power lines of three phases are connected to the intermediate terminals U2, V2, W2. (This formation will be referred to as "high speed winding formation".)
According to another known method, as shown in FIGS. 8a and 8b, intermediate terminals X2, Y2, Z2 are provided at intermediate portions of three windings for three phases. For low speed rotation, as shown in FIG. 8a, terminals X, Y, Z provided at one-side ends of the three windings for three phases are connected together while terminals U, V, W provided at the other ends of the windings are connected to power lines of three phases, respectively, to thereby form a Y-connection in which the three entire windings for three phases are used. (This formation will be referred to as "low speed winding formation".) (It is to be noted that also in FIG. 8, used parts of windings are depicted with bold lines.) For high speed rotation, as shown in FIG. 8b, the terminal U and the intermediate terminal Z2, the terminal V and the intermediate terminal X2, and the terminal W and the intermediate terminal Y2 are connected respectively while power lines of three phases are connected to the terminals U, V, W, respectively, to thereby form a .DELTA.-connection in which only part of each winding for each phase is used. (This formation will be referred to as "high speed winding formation".)
As described above, characteristics of a motor are changed by changing the number of turns for each phase. As understood from the relation between turns ratio and power curves as shown in FIG. 9, in such switching between the low and high speed winding formations, the larger the turns ratio is, the larger torque can be produced with low speed rotation and the higher power can be obtained with high speed rotation.
It is to be noted that in FIG. 9, M denotes the relation where the turns ratio is 1:M.
In the switching method as shown in FIG. 6, the turns ratio of the .DELTA.-connection to the Y-connection is 1 to 3(M=3) and cannot be changed.
In the switching methods as shown in FIGS. 7a, 7b and FIGS. 8a, 8b, the turns ratio can be arbitrarily determined by choosing the number of turns which are used in the high speed winding formation. (This number of turns depends on positions at which the intermediate terminals are provided). However, when voltage is applied to the parts of windings to be used in high speed winding formation (depicted with bold lines), voltage is induced at the parts of windings not to be used. At that time, if the sum of the input voltage and the induced voltage exceeds the insulation limit voltage, an insulation defect is caused between the windings. Therefore, there exist the maximum turns ratio and the maximum input voltage which ensure that the insulation limit voltage is not exceeded. Thus, the maximum turns ratio is determined of itself.
Further, even if the turns ratio can be large, there is a fall in power characteristic at the time of switching from the low speed winding formation to the high speed winding formation as shown in FIG. 9, which hinders obtaining a high power characteristic in a wide range.
In a machine tool such as a machining center, it is desired that low-speed heavy cutting is possible, and rotating speed of a spindle can be increased to improve the efficiency of cutting work. Thus, a spindle motor for driving a spindle is required to have a larger torque output at a low speed, to have a higher speed, and to have higher power. In order to meet these requirements by switching the winding formation, it is necessary to secure a large turns ratio, and to remove an influence of induced voltage. In addition to securing a large turns ratio, smooth switching from the low speed winding formation to the high speed winding formation which causes only a small fall in power characteristic and does not invite a shortage of output torque is also desired.